1. Field of the Invention
This invention relates to the protection of workers on nuclear reactors and in particular, the protection of workers in the area of the nuclear reactor head.
Typically, nuclear reactors have a closure, called a head, covering the top end of the vessel which is bolted to the vessel by means of bolts passing through holes in a peripheral flange thereof. The head is usually spherically shaped above the peripheral flange with reactor control drive mechanisms mounted thereon within the top central area thereof. Reactor head lifting members in the form of stiff vertically extending rods, typically three equally spaced about the top, are located between the control drive mechanisms and the head securing bolts. The lifting members are secured to the head by integral bosses for lifting the head closure on or off the vessel when the head bolts are removed.
The top central area containing the reactor control drive mechanisms is significantly more radioactively dangerous to personnel in the area of the head bolts than the rest of the head area and the vessel because the control mechanisms are in communication with the reactor core. Often, a cylindrical shroud is provided between the lifting members and the control mechanisms of the top central area of the head, but these cannot provide protection of sufficient magnitude to the personnel working with the head bolts or in the vicinity thereof.
During refueling operations, the reactor vessel head must be removed to access the fuel. In order to do this, workers must remove the holddown bolts. Removal of these bolts requires appreciable time, thus causing the workers to expose themselves to radiation from the shroud area where the highly radioactive control rod drives are located. In addition, any work or maintenance that is performed around the head after it is stored on the operating deck will expose the workers to this radiation field. A system is needed that will provide and allow for the following:
1.) Shielding that can be attached and removed from the vessel head in a short time to minimize exposure; PA1 2.) Shielding that will provide radiation protection without interfering with access to closure head holddown bolts; and, PA1 3. Shielding that will allow additional thickness to be added to cover "hot spots".